In this technical field, research and development for next-generation mobile communication schemes are accelerated. 3rd Generation Partnership Project (3GPP) being a standardization organization for Wideband Code Division Multiple Access (W-CDMA) is considering LTE (Long Term Evolution) as a successor communication scheme for the W-CDMA, HSDPA and/or HSUPA. In the LTE, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and a SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme will be used as a downlink radio access scheme and an uplink radio access scheme, respectively. For example, see 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006.
The OFDM scheme is a multi-carrier scheme where a frequency band is segmented into smaller frequency bands (subcarriers) and data is transmitted over the individual frequency bands. According to the OFDM scheme, the subcarriers are densely arranged and partially overlapped without mutual interference, which can achieve faster transmissions and improve frequency utilization efficiency.
The SC-FDMA scheme is a single-carrier scheme where a frequency band is segmented into several frequency subbands and multiple terminals use the different frequency subbands for transmissions to reduce interference between the terminals. According to the SC-FDMA scheme, transmit power has smaller variations, which can reduce power consumption at the terminals and achieve broader coverage.
The LTE is a communication system where both one or more uplink physical channels and one or more downlink physical channels are shared among multiple user apparatuses. These channels shared among the multiple user apparatuses are generally referred to as shared channels. In the LTE, uplink communications are conducted in PUSCHs (Physical Uplink Shared Channels) while downlink communications are conducted in PDSCHs (Physical Downlink Shared Channels).
In a communication system using the shared channels, it is necessary to signal to which user apparatuses the shared channels are to be assigned for each subframe (1 ms in the LTE). In the LTE, control channels used for this signaling are referred to as PDCCHs (Physical Downlink Control Channels) or DL (Downlink)-L1/L2 Control Channels. For example, the PDCCH may include some information items such as downlink scheduling information, acknowledgement information (ACK/NACK), uplink scheduling grant, overload indicators and transmit power control command bits. For example, see R1-070103, Downlink L1/L2 Control Signaling Channel Structure Coding.
The downlink scheduling information and the uplink scheduling grant are used to signal to which user apparatuses the shared channels are to be assigned. For example, the downlink scheduling information may include information items for downlink shared channels such as assignment information of downlink resource blocks (RBs), IDS for UEs, the number of streams in multiple input multiple output (MIMO), precoding vector information, data sizes, modulation schemes and HARQ (Hybrid Automatic Repeat reQuest) information. On the other hand, the uplink scheduling grant may include information items for uplink shared channels such as assignment information of uplink resource blocks, IDS for UEs, data sizes, modulation schemes, uplink transmit power information and demodulation reference signal information in uplink MIMO.
A MIMO (Multiple Input Multiple Output) scheme is a multi-antennas type communication designed for faster and higher quality signal transmission by using multiple antennas. In addition, directivity controlled beams can be transmitted to communication opponents by duplicating a transmitted signal stream and combining the duplicated signals streams with appropriate weights. This is referred to a precoding scheme, and the applied weights are referred to a precoding vector or more generally are referred to a precoding matrix.
FIG. 1 schematically illustrates an exemplary precoding operation. Each of two streams (transmitted signals 1, 2) is duplicated into two streams at duplication units, and each of the two streams is multiplied with a precoding vector. Then, the streams are combined and transmitted as illustrated. The precoding is classified into a closed-loop scheme and an open-loop scheme. In the closed-loop scheme, the precoding vector is adaptively controlled to have appropriate values based on feedbacks from the receiver side (user apparatus) unlike the open-loop scheme. In FIG. 1, an exemplary closed-loop operation is illustrated. In the precoding scheme, individual streams are separately transmitted in space, which can greatly improve quality of the individual streams.
Meanwhile, a technique referred to as delay diversity or cyclic delay diversity (CDD) is proposed. In this technique, a number of duplications corresponding to the number of antennas are generated for a signal to be transmitted, and different path delays from the duplication units to the antennas are set for the duplicated signals. Since the same signal is transmitted at different timings, the technique is preferred for achieving uniform quality of the signal over different streams.
As illustrated in FIG. 2, the same signals are transmitted from multiple antennas at different timings. The receiver side receives the signals as several paths and combines them, which can lead to diversity effect.
In addition, the precoding may be combined with the CDD to gain quality improvement by both the precoding and the CDD. In this case, transmitted signals may have different characteristics depending on whether signal processing for the precoding or signal processing for the CDD is carried out prior to the other.
FIG. 3 schematically illustrates exemplary signal processing for the CDD after signal processing for the precoding has been carried out. FIG. 4 illustrates components in FIG. 3 in detail. In this illustration, NFFT represents a fast Fourier transform (FFT size, τ represents an amount of delay, and Skn represents the n-th stream of the k-th subcarrier.
As illustrated in FIG. 4, operations on individual signal components are represented in a matrix form, and a matrix operation (Dk) for the CDD and a matrix operation (F) for the precoding are not commutative in general. For this reason, among the two successively conducted signal operations, the latter may more significantly affect signals to be transmitted. In the illustrated example, the signal quality improvement effect by the CDD is brought out more significantly, and thus there is a higher likelihood that the signal quality may be averaged over transmitted streams. On the other hand, the signal quality improvement effect by the precoding may be weakened. This scheme may be advantageous to the open-loop type precoding in that a precoding vector is fixed. In FIG. 4, the precoding vector is denoted as “F” and works as a weighting factor. This type of scheme is disclosed in 3GPP R1-070236, “Precoding for E-UTRA downlink MIMO”, LG Electronics, Samsung and NTT-DoCoMo, for example.
FIG. 5 schematically illustrates exemplary signal processing for the precoding after signal processing for the CDD has been carried out. FIG. 6 illustrates components in FIG. 5 in detail. Also in this case, the latter signal processing may more significantly affect a signal to be transmitted. Thus, in the illustrated example, the signal quality improvement effect by the precoding is brought out more significantly and may be advantageous to the quality improvement for individual streams. On the other hand, the signal quality improvement effect by the CDD may be weakened. For this reason, it is preferable that precoding vectors be controlled adaptively. In FIG. 5, the precoding vectors are denoted as “Ui”. From a reserved set of P precoding vectors {U1, U2, . . . , UP}, an optimal vector Ui determined based on a feedback signal from a communication opponent is selected adaptively. In other words, this scheme is advantageous to the closed-loop precoding. This type of scheme is disclosed in 3GPP R1-072461, “High Delay CDD in Rank Adapted Spatial Multiplexing Mode for LTE DL”, Ericsson, for example.